This invention relates to the thermoforming of shaped articles and, more particularly, to an improved apparatus for thermoforming shaped articles using a gas fed catalytic heater as a heat source.
Shaped plastic articles such as plastic drinking cups, party plates, video jackets and the like are commonly made by a thermoforming process wherein a sheet of thermoplastic material is heated to its softening point and then shaped in a mold to form the desired articles. Typically, the sheet of thermoplastic material is fed from a continuous supply roll through a heating zone and thence onto to the mold where the articles are formed, the process being commonly referred to in the industry as a "Roll Fed In-Line Thermoforming Process".
Heretofore, electric infrared devices have been employed in the thermoforming process to heat the sheet of thermoplastic material as it is being fed from the supply roll to the mold. The infrared heaters have been controlled by means of percentage timers which turn the heaters on for a percentage of the time and then off for the remainder. The operator of the equipment must frequently adjust the percentage timers to offset the effects of convected heat and thus produce acceptable molded parts. The ratio of infrared to convected heat produced by these devices is typically on the order of about 50/50. Because of this rather large ratio and the uncertain effects of convective heat on the heating of the thermoplastic sheet, attempts to automatically control the temperature of the sheets to a predetermined value have not been successful. It has been proposed, for example, to employ a closed loop temperature control system for sensing the temperature of the sheet and then controlling the temperature by regulating the power input to the electric heaters but the results have been highly erratic. Since the sheet is being heated by both infrared and convective heat, it is difficult, if not impossible, to determine which source is predominant in the heating of the plastic material.
Another problem with electric infrared heaters is that they emit short wave infrared energy, that is, infrared radiation having a wave length of from about 1.0 to about 3.0 microns. Unfortunately, most plastic materials do not readily absorb short wave infrared radiation.
Another serious problem is that electric infrared heaters operate at fairly high temperatures and will ignite most flammable materials upon contact. This is particularly troublesome in the operation of roll fed in-line thermoforming equipment wherein occasionally stray pieces of flammable thermoplastic sheet inadvertently contact the surface of the heaters creating a serious fire hazards.
It is therefore an important object of the invention to provide an improved apparatus for thermoforming shaped articles from a sheet of thermoplastic material.
Another object of the invention is to provide an improved thermoforming apparatus which utilizes as a heat source an infrared heating device which produces only minimal amounts of convective heat.
Still another object of the invention is to provide such an improved thermoforming apparatus employing a closed loop temperature control system.
A more specific object of the invention is to provide such an improved thermoforming apparatus utilizing an infrared heating device emitting infrared energy which is readily absorbed by most plastic materials.
Still another specific object of the invention is to provide such an improved thermoforming apparatus utilizing an infrared heating device which operates at relative low temperatures and which is flameless and safe.